Bottom mounted pedical screw assembly and method of use

ABSTRACT

A bone screw, which can be a pedicle screw, is generally tulip-shaped. A bore in the bottom of the tulip permits a threaded shaft to be partially inserted into the open interior of the tulip. A seat is formed on the threaded shaft and receives the bottom of the tulip, and prevents the threaded shaft from being inserted too far into the tulip. A saddle is positioned in the interior of the tulip and receives the end of the threaded shaft on one end and can include a trough on its opposite face which can receive a rod that connects together two or more such assemblies. A retaining screw threads into the upper interior surface of the tulip to hold the subcomponents together.

This application claims priority under 35 U.S.C. § 119 to U.S.provisional application No. 61/027,876, filed 12 Feb. 2008, the entiretyof which is incorporated by reference herein.

BACKGROUND

1. Field of Endeavor

The present invention relates to medical devices and processes used inbone fixation, and more specifically to bone screws.

2. Brief Description of the Related Art

Bone screws, including pedicle screws, have been used for years toprovide an anchor into bone in order to secure other devices to thebone. While advances have been made in such bone screws, current designsdo not accommodate all uses and anatomy. For example, U.S. Pat. No.5,207,678, issued to Harms et al., describes a bone screw assembly thatrequires the bone screw itself to be assembled with other parts of thedevice prior to driving the bone screw into bone. While thisconfiguration has been quite popular, in some circumstances it can bedesirable to position the bone screw itself in the desired location inbone, and thereafter assemble the remaining elements of the bone screwassembly. Because the bone screw assembly described in the '678 patentincludes a top-mounted screw, that is, one in which the bone screw 2must be inserted through the top of its housing 6 before driving thescrew 2 into bone, the practitioner must drive the bone screw with thehousing 6 loosely attached to the proximal end of the screw, thuspotentially interfering with driving the screw and obstructing thepractitioner's view of the screw head and the bone insertion point.

There remains a need for bone screws which can be used to anchor otherdevices to bone which address, and even overcome, some or all of thedeficiencies in the prior art devices.

SUMMARY

According to a first aspect of the invention, a polyaxial bone screwassembly comprises a generally cup-shaped tulip having a top opening, abottom bore, a semi-spherical inner bottom surface, and a semi-sphericalouter bottom surface, a saddle sized to be received in the tulip, thesaddle having a semi-spherical outer bottom surface and a recessextending upwardly from said bottom surface, a bone screw having adistal end and a proximal end, a threaded shaft extending proximallyfrom the distal end, a laterally extending platform positioned distal ofthe proximal end, and a post extending proximally of the platform, theplatform including a semi-spherical proximal surface, wherein the bonescrew post and the saddle include a snap fit, the snap fit including arecess in one of the bone screw post and the saddle and an enlargementon the other of the bone screw post and the saddle, the enlargementreceived in the recess.

According to another aspect of the present invention, a method ofinstalling a bone screw assembly in a bone comprises providing a bonescrew assembly including a threaded bone screw having a laterallyextending platform, a cup-shaped tulip having a proximal opening and adistal opening, and a saddle having a distal bore, moving the bone screwinto the bone, positioning the tulip over a proximal portion of the bonescrew and proximally of the platform, positioning the saddle inside thetulip, and securing the saddle to said proximal portion of the bonescrew.

Still other aspects, features, and attendant advantages of the presentinvention will become apparent to those skilled in the art from areading of the following detailed description of embodiments constructedin accordance therewith, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention of the present application will now be described in moredetail with reference to exemplary embodiments of the apparatus andmethod, given only by way of example, and with reference to theaccompanying drawings, in which:

FIG. 1 illustrates a longitudinal cross-sectional view of proximalportions of an exemplary bone screw assembly; and

FIG. 2 illustrates a longitudinal cross-sectional view of proximalportions of a second exemplary bone screw assembly.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to the drawing figures, like reference numerals designateidentical or corresponding elements throughout the several figures. Theexemplary embodiment illustrated in FIG. 1 will first be discussed. Inthe drawing figures, the term “proximal” will refer to the portions ofthe devices towards the top of the drawings, and the term “distal” willrefer to the portions of the devices towards the bottom of the drawings.Unless otherwise stated, the devices are rotationally symmetrical abouta longitudinal axis that extends between the proximal and distal ends ofthe devices.

10 designates the entire bottom mounted pedicle screw assembly of thefirst embodiment.

12 designates the bone screw portion per se of the assembly 10, whichextends distally of the assembly.

14 designates the proximal ‘tulip’ portion of the assembly 10. The tulipis generally tulip-shaped with two laterally opposed slots in thesidewalls, in the same manner as currently commercially availablepedicle screw assemblies; those slots are out of the plane of thesectional view of FIG. 1 and can therefore not be seen in the figure.The slots advantageously are open-ended and extend distally from theproximal end of the tulip 14 toward a distal closed end. The rod 20,described below, extends laterally through those slots.

16 designates the saddle portion of the assembly 10, which alsofunctions to transfer load between the rod (from the retaining and loadscrew 18, described below) to the lower portions of the tulip 14. Thesaddle 16 is preferably rotationally symmetrical about the longitudinalaxis of the assembly 10, except for the provision of an optionallaterally extending trough 48 configured to receive the rod 20.

18 designates the retaining and load screw, which is externally threadedand mates with corresponding threads on the interior, upper surface ofthe tulip 22, in a manner similar to currently commercially availablepedicle screw assemblies. Alternatively, the screw 18 can be aninternally threaded nut, and the tulip 14 can alternatively includemating threads on its upper exterior surface.

20 designates a transversely extending rod that is clamped between thescrew 18 and the saddle 16, and which extends between and thereforejoins together two or more assemblies 10 in a manner well known to theroutineer in the art.

22 designates the thread on the interior surface of the tulip 14.

24 designates a semi-spherical concave interior seat surface of thetulip 14, against which the correspondingly shaped, convex exteriorsurface of the adjacent saddle 16 bears when the saddle 16 is pressedagainst the bottom interior surface of the tulip.

26 designates a semi-spherical convex exterior bearing surface of thetulip 14, against which the correspondingly shaped, concave interiorsurface of the adjacent platform 36 of the bone screw 12 bears.

28 designates a bore through the bottom of the tulip 14, through whichthe post 34 of the bone screw 12 extends. The radial or lateral size ofthe bore 28 is at least large enough for the post 34 to be insertedthrough the bore from the bottom of the tulip 14. The radial size of thebore 28 is selected to permit the assembly 10 to be polyaxial, that is,to permit the tulip to pivot about the longitudinal axis of the bonescrew 12 while still being clamped between the saddle 16 and theplatform 36.

30 designates the inner surface of the tulip 14 which defines the bore28.

32 designates the threaded shaft of the bone screw 12, the threads perse not being illustrated so as to not obscure aspects of the invention.Also not illustrated is the pointed distal end of the bone screw 12.

34 designates the proximal post of the bone screw 12, which extendsproximally from the platform 36. The post 34 has a longitudinal lengthsufficient to extend through the bore 28 and into a recess 42 in thesaddle 16 when the bone screw 12 is positioned partially inside thetulip and the platform 36 bearing against the distal face 26 of thetulip.

36 designates the platform of the bone screw 12 against which the lowerportions of the tulip 14 bear and are clamped. The platform isillustrated as being generally cup-shaped; however, the distal surfacescan be rounded, as illustrated, squared, or any other shape. The lateralsize of the platform, that is, the distance between the outer lateraltip of the platform to the shaft 32, is selected in conjunction with thesize of the bore 28, the lateral cross-sectional radius of the post 34,and other dimensions of the assembly, to maximize the amount of thetulip 14 that is clamped between the saddle 16 and the platform, for apredetermined maximum pivot angle for the tulip to function as apolyaxial assembly. In the illustration of FIG. 1, which is otherwisegenerally not to scale, an angle a of between 10 and 15 degrees of pivotis illustrated. Optionally, the platform can be formed of at least threelaterally extending arms, rather than a circumferentially continuousstructure.

38 designates a semi-spherical, upper, concave surface of the platform36, against which the correspondingly shaped, convex lower exteriorsurface of the adjacent tulip 14 bears.

40 designates an enlargement at the proximal end of the post 34. Theenlargement 40 cooperates with lower portions of the saddle 16 to snapand retain the saddle onto the post, while also optionally permittingthe saddle to freely rotate about the longitudinal axis of the post,after the post has been inserted through the bore 28. While illustratedto include a rectangular, laterally extending lip, the enlargement 40can take any shape that permits it to snap fit with the saddle, and tobe inserted through the bore 28 from the bottom of the tulip 14.

42 designates a recess in the center of the saddle 16, into which theenlargement 40 is snapped to retain the saddle to the post 34 and,therefore, the tulip 14 sandwiched between the platform 36 and thesaddle. The longitudinal depth of the recess 42 is selected so that theenlargement 40 does not interfere with the saddle 16 moving downward(distally) relative to the tulip 14, to clamp the tulip between thesaddle and the platform. Preferably, the recess 42 extendslongitudinally completely through the saddle, making it a through borewith upper (proximal) and lower (distal) openings, to permit a user toinsert a torque driver (e.g., a screwdriver) through the saddle and intoengagement with portions of the post 34, so that the bone screw 12 canbe driven into bone by the practitioner, if the assembly 10 ispreassembled with the bone screw 12, tulip 14, and saddle 16 alreadysnapped together.

44 designates a blind bore in the upper (proximal) portion of the post34, which includes a non-circular inner surface configured so that atorque driver can be inserted into the bore 44 to rotate andlongitudinally drive the bone screw into bone.

46 designates a semi-spherical convex lower exterior bearing surface ofthe saddle 16, against which the correspondingly shaped, concaveinterior surface of the adjacent tulip 14 bears.

48 designates an optional trough in the upper surface of the saddle 16,in which the rod 20 is positioned and is clamped by the saddle and thescrew 18. While the exact shape of the trough is not critical, a concaveshape as illustrated is preferred; a V-notch could also easily andexpeditiously be used. The trough is sized to receive the rod 20.

50 designates one or more optional reinforcements between the platform36 and the bone screw 12.

52 designates a blind bore in the upper (proximal) surface of the screw18, configured to receive a torque driver in much the same manner asbore 44.

54 designates a radially inwardly extending lip on the inner surface ofthe recess 42. The lip 54 is configured to act with the enlargement 40on the post 34 to form a snap fit between the saddle and the post. Thelip 54 can optionally be circumferentially continuous, or can becircumferentially discontinuous in the form of at least three fingersthat extend radially inwardly from the saddle 16.

56 designates the threads on the outer surface of the screw 18.

In the exemplary embodiment illustrated in FIG. 2, in which the tulip 16and platform 36 are not illustrated:

58 designates a saddle similar in most respects to the saddle 16, exceptthat it includes a downwardly (distally) extending post 60 having anenlargement 62.

60 designates the post of the saddle 58.

62 designates the enlargement of the post 60.

64 designates the post of the bone screw 12.

66 designates a blind bore in the proximal end of the post 64, sized toreceive the post 60 therein to form a snap fit, while being deep enoughso that the saddle 58 can move downward (distally) as described above.

68 designates an enlarged portion of the bore 66, sized to receive thepost 60 and the enlargement 62 to form a snap fit. As with theembodiment illustrated in FIG. 1, the structures which form the snap fitcan take a number of forms. As with the embodiment of FIG. 1, theembodiment of FIG. 2 includes bores similar to bores 42, 44, to permit atorque driver to be inserted through the saddle 58, post 60, and intothe post 64, to permit the practitioner to drive the bone screw intobone.

With reference to FIG. 1, exemplary methods of using a bone screwassembly will now be described. The practitioner identifies a locationin bone, e.g., a pedicle of a patient's spine, in which it is desired toinstall a bone screw. The bone screw 12 is positioned at that location,with a torque driver (not illustrated) engaged in the bore 44, and thebone screw is driven into the bone. After the bone screw 12 is in place,at least partially, in the bone, the tulip 14 is positioned on thescrew, with the post 34 extending through the bore 28. The saddle 16 isthen positioned inside the tulip. In order to secure the saddle 16 tothe post 34, a distally directed force must be applied to the saddle, sothat the lip 54 can ride over the enlargement 40; this force can besupplied by the screw 18, or can be applied by the practitioner directlyto the proximal surfaces of the saddle.

Once the saddle 16 and screw 12 have been secured together, the tulip 14will have been captured between them, and more specifically between theplatform 36 and the saddle. Because of the cooperating semi-sphericalsurfaces of the saddle, tulip, and platform, the tulip 14 can pivotabout the longitudinal axis of the screw 12 while still retained on thescrew, thus permitting the assembly 10 to be polyaxial.

With the saddle 16 attached to the post 34, the cylindrical bar 20,known to the routineer in the art and useful to connect together atleast two, e.g., pedicle screw assemblies, is positioned over theproximal surface of the saddle, advantageously in the optional trough48, and the retaining screw 18 is put in place. Distal motion of theretaining screw 18 causes the retaining screw to bear distally on theproximal portion of the bar 20, causing the bar 20 to be forced distallyagainst the saddle 16, which in turn is free to move distally over thepost 34 because of the longitudinal size of the bore 42 and bear againstthe surface 24 of the tulip 14, which is turn causes the tulip to beardown distally against the surface 38 of the platform 36. Because theplatform 36 is secured to bone, the force generated by the retainingscrew 18 causes the tulip 14 to be clamped between the saddle and thescrew 12, while still permitting the tulip to assume angled orientationsrelative to the screw 12.

Optionally, the saddle 16, tulip 14, and screw 12 can be pre-assembled,that is, the saddle and post 34 secured together with the tulip trappedbetween them, before the practitioner drives the screw 12 into bone. Inthis alternative, the practitioner's torque driver is positioned in athroughbore 42 in the saddle to drive the screw 12.

With reference to FIG. 2, the same general process is used to mount theassembly to bone as that used with the embodiment of FIG. 1, with thepractitioner's torque driver used to drive the bone screw 12 at post 64,optionally through the saddle and the post 60.

While the invention has been described in detail with reference toexemplary embodiments thereof, it will be apparent to one skilled in theart that various changes can be made, and equivalents employed, withoutdeparting from the scope of the invention. The foregoing description ofthe preferred embodiments of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Theembodiments were chosen and described in order to explain the principlesof the invention and its practical application to enable one skilled inthe art to utilize the invention in various embodiments as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto, and theirequivalents. The entirety of each of the aforementioned documents isincorporated by reference herein.

1. A polyaxial bone screw assembly comprising: a generally cup-shapedtulip having a top opening, a bottom bore, a semi-spherical inner bottomsurface, and a semi-spherical outer bottom surface; a saddle sized to bereceived in the tulip, the saddle having a semi-spherical outer bottomsurface and a recess extending upwardly from said bottom surface; a bonescrew having a distal end and a proximal end, a threaded shaft extendingproximally from the distal end, a laterally extending platformpositioned distal of the proximal end, and a post extending proximallyof the platform, the platform including a semi-spherical proximalsurface; wherein the bone screw post and the saddle include a snap fit,the snap fit including a recess in one of the bone screw post and thesaddle and an enlargement on the other of the bone screw post and thesaddle, the enlargement received in the recess.
 2. The bone screwassembly according to claim 1, further comprising a retaining elementhaving a thread, and wherein the tulip further comprises a threadconfigured and arranged to mate with the retaining element thread. 3.The bone screw assembly according to claim 2, wherein the retainingelement comprises a retaining screw, the retaining screw threadcomprises an external thread, the tulip comprises a proximal innersurface, and the tulip thread comprises a thread on the tulip proximalinner surface.
 4. The bone screw assembly according to claim 1, furthercomprising a bar extending laterally through the tulip between theretaining screw and the saddle.
 5. The bone screw assembly according toclaim 1, wherein the saddle recess comprises a blind bore.
 6. The bonescrew assembly according to claim 1, wherein the saddle recess comprisesa throughbore.
 7. The bone screw assembly according to claim 1, whereinthe snap fit recess is in the saddle and the snap fit enlargement is onthe bone screw post.
 8. The bone screw assembly according to claim 1,wherein the snap fit recess is in the bone screw post and the snap fitenlargement is on the saddle.
 9. The bone screw assembly according toclaim 1, wherein the bone screw platform is laterally larger than thetulip bottom bore.
 10. The bone screw assembly according to claim 1,wherein the tulip bottom bore and the bone screw post are mutually sizedso that the tulip can pivot relative to the bone screw.
 11. The bonescrew assembly according to claim 1, wherein the tulip bottom bore andthe bone screw post are mutually sized so that the tulip can pivotrelative to the bone screw up to about 15 degrees.
 12. A method ofinstalling a bone screw assembly in a bone, the method comprising:providing a bone screw assembly including a threaded bone screw having alaterally extending platform, a cup-shaped tulip having a proximalopening and a distal opening, and a saddle having a distal bore; movingthe bone screw into the bone; positioning the tulip over a proximalportion of the bone screw and proximally of the platform; positioningthe saddle inside the tulip; and securing the saddle to said proximalportion of the bone screw.
 13. The method according to claim 12, furthercomprising: pivoting the tulip relative to the bone screw.
 14. Themethod according to claim 12, wherein securing comprises snappingtogether.
 15. The method according to claim 12, wherein said movingprecedes said positioning the saddle.
 16. The method according to claim12, wherein said positioning the saddle precedes said moving.
 17. Themethod according to claim 12, wherein said providing further comprisesproviding a bar and a retaining element, and further comprising:positioning the bar proximal of the saddle; positioning the retainingelement proximal of the bar; and clamping the bar between the retainingelement and the saddle, including forcing the retaining element distallyagainst the bar.
 18. The method according to claim 17, wherein: saidproviding a bar and a retaining element precedes said securing; and saidforcing further comprises forcing until the saddle and the bone screware secured together.
 19. The method according to claim 17, whereinclamping comprises clamping until the tulip is clamped between thesaddle and the platform.
 20. The method according to claim 19, furthercomprising: pivoting the tulip relative to the bone screw before saidclamping.